|PR-10 genes in apple seedlings (Malus domestica): identification, characterization and spatio-temporal expression after induction by acibenzolar-S-methyl (ASM), a functional analogue of salicylic acid.
Production of PR proteins is one of the most important defence responses deployed by plants under different stresses (biotic and abiotic). Fourteen families are currently classified, among which the PR-10 family whose function remains unknown. This family includes a large number of acidic PR proteins presenting similarity in molecular weight (16-18 kDa), amino acid sequences and gene structure. Their intracellular localisation and their homology to pollen and food allergens such as Bet v 1, the major birch pollen allergen and Mal d 1, the major apple allergen, constitute two specific features of this PR family.
This study is focusing on the identification of PR-10 genes of apple seedlings (Malus domestica) and their expression after application of various abiotic stresses including treatment with acibenzolar-S-methyl (ASM), a functional analogue of salicylic acid.
Complete sequence of four cDNAs (named AP1 to AP4) was identified in the young leaves of apple seedling (cv. Golden Delicious) and the corresponding genes were grouped into two subclasses APa, including AP1 and AP4 and APb, regrouping AP2 and AP3. These two subclasses were differentiated both at the protein level by numerous amino acid exchanges and at the nucleotide level by the presence of an intron in members of the APb subclass only. Scanning of the 5’ regulating regions of AP1 to AP4 genes for putative binding sites for transcription factors, revealed the presence of numerous motifs involved in response to biotic and abiotic responses. The analysis of expression of the APa and APb genes was carried out at the transcriptional and translational level after treatment by ASM. Results show a significant induction of transcripts and proteins after application of ASM. Western blot analysis revealed the presence of two polypeptides of 17 and 18 kDa at 48 h after treatment by ASM. In situ localisation of PR-10 products in the young leaves of apple seedlings showed that they are mainly localised in palisade parenchyma and in vascular tissues.
Contrary to the APb genes, members of the APa subclass were considered as yet undescribed apple PR-10 genes. Our results suggest that the apple PR-10 genes, in particular those of the APa subclass, which are strongly inducible by ASM, could play a role in local and systemic defences.
Key-words: PR-10 proteins; Malus domestica cv. Golden Delicious; Induction of resistance; Defence mechanism; Abiotic stresses, Acibenzolar-S-methyl; Allergens.